Method and arrangement for signal modulation

ABSTRACT

The invention relates to a method and arrangement for modulating a signal to be transmitted, the arrangement comprising an encoder and a frequency modulator. In order to enable high rate transmission in a flexible manner in a narrow frequency band, the encoder (104) is a differential encoder and before the frequency modulator, the arrangement comprises means for multiplying the signal to be transmitted by a factor of the form pi/(2m), where m is a positive integer greater than one.

FIELD OF THE INVENTION

The invention relates to a method for modulating a signal, the methodusing continuous phase modulation and comprising encoding and frequencymodulation of a signal to be transmitted.

BACKGROUND OF THE INVENTION

A modulation method used on a transmission path is a significantparameter when new data transmission systems are developed. Because oflosses occurring on the transmission path and because of transmissionpath capacity, data symbols to be transmitted cannot be transmitted overthe transmission path as such, but the symbols must be modulated using asuitable method so as to obtain good transmission path capacity andtransmission quality.

The bandwidth required by transmission is a significant factorparticularly in radio systems. The aim is to achieve maximumtransmission capacity by using a narrow bandwidth. On the other hand,the aim is to provide a transmitter and a receiver as easily andadvantageously as possible. In radio systems, the aim is generally touse a modulation method having a constant envelope, because a C-classamplifier solution can then be used. The C-class amplifiers are simplein structure and advantageous in efficiency. This is particularlyrelevant as far as terminal power consumption is concerned.

There are several prior art modulation methods having a constantenvelope, including Minimum Shift Keying MSK, Gaussian Minimum ShiftKeying GMSK, Tamed Frequency Modulation TFM and Continuous PhaseModulation CPM. The GMSK method is used in the GSM cellular radiosystem. It has a narrow frequency spectrum and high performance, whereasdata transmission rates are not very high. The coded CPM methods usuallyhave a narrow frequency spectrum and high performance, making high datarates possible. However, equipments required become complex instructure, for which reason these methods have not been used in priorart systems.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to provide a method and arrangementimplementing the method, enabling high data rate transmission in anarrow frequency band without complex equipments required. This isachieved by the method of the type described in the introduction, whichis characterized in that the encoding performed is differential encodingand that a signal to be transmitted before the frequency modulation ismultiplied by a factor of the form π/(2m), where m is a positive integergreater than one.

The invention also relates to an arrangement for modulating a signal tobe transmitted, the arrangement comprising an encoder and a frequencymodulator. The arrangement of the invention is characterized in that theencoder is a differential encoder and that before the frequencymodulator, the arrangement comprises means for multiplying the signal tobe transmitted by a factor of the form π/(2m), where m is a positiveinteger greater than one.

The preferred embodiments of the invention are disclosed in thedependent claims.

The method and arrangement of the invention provide many advantages. Thenumber of feasible states on a unit circle can be minimized by using theselected factor, thus providing a receiver of reduced complexity. On theother hand, bit error ratio obtained by the method of the invention,indicating the ability of the system to tolerate errors, is higher thanin GMSK, but if the transmission band remains unchanged, the data ratesobtained are substantially higher. Consequently, the method of theinvention enables more data to be transmitted using the same frequencyband. As compared with the coded CPM methods, the equipments needed, thetransceiver in particular, are substantially easier to implement.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail bymeans of preferred embodiments with reference to the accompanyingdrawings, in which

FIG. 1 is a block diagram illustrating a first example of thearrangement of the invention,

FIG. 2 is a block diagram illustrating a second example of thearrangement of the invention,

FIG. 3 is a block diagram illustrating a third example of thearrangement of the invention,

FIG. 4 shows an example of a feasible state diagram of the modulationmethod of the invention, and

FIG. 5 shows an example of a feasible phase trajectory of the modulationmethod of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Let us first study an example of an advantageous structure of thearrangement of the invention by means of a block diagram presented inFIG. 1. The figure presents a radio system terminal structure essentialto the invention. Naturally, in order to function, an apparatus to beimplemented must also include other components apart from thosepresented in FIG. 1, as it is obvious to those skilled in the art.However, for the sake of clarity, they are not dealt with in the figureand description.

The arrangement comprises a data source 100 generating a digital signal102 to be transmitted. The data source may be a microphone connected toa speech encoder, for example. In that case, the signal to betransmitted comprises speech in digital form. Other data sources mayinclude a computer or a modem, for example. Let us assume herein thatthe signal to be transmitted is composed of data symbols d_(i)=[0,1].Furthermore, let us assume that a symbol rate is 1/T, where T is thelength of the data symbol. In the arrangement of the invention, thesignal 102 is first applied to a differential encoder 104 differentiallyencoding each data symbol d_(i)=[0,1]. The output of the differentialencoder thus includes the following symbols:

{circumflex over (d)} _(i) =d _(i) ⊕d _(i−1)

where ⊕ denotes modulo 2 addition. The encoded symbols are of the form 0or 1. The values so obtained are further applied to converter means 106performing conversion in which symbols [−1, 1] represent the symbols[0,1]. In other words, the output of the converter means includes valuesα_(i)=1−2{circumflex over (d)}_(i), where αε{−1,1}.

In a preferred embodiment of the invention, the symbols so obtained areapplied to a filter 108 filtering the signal in accordance with aspectral pattern desired. A transfer function following the Gaussiandistribution can be preferably selected as the transfer function of thefilter. In that case, the transfer function can be defined in the form${g(t)} = {{h(t)} \otimes {{rect}\left( \frac{t}{T} \right)}}$

where t stands for time, ⊕ indicates convolution, and a function rect(x)is defined by${{rect}\left( \frac{t}{T} \right)} = {{\frac{1}{T}\quad {when}\quad {t}} < \frac{T}{2}}$${{rect}\left( \frac{t}{T} \right)} = {0\quad {{otherwise}.}}$

When the Gaussian distribution is used, a function h(t) can be selectedby${h(t)} = \frac{^{(\frac{- t^{2}}{2\sigma^{2}T^{2}})}}{\sqrt{2\pi}\sigma \quad T}$

where $\sigma = \frac{\sqrt{\ln (2)}}{2\pi \quad {BT}}$

and BT=β.

Herein, B stands for a 3-dB bandwidth of the filter with the impulseresponse h(t) and T is thus the length of the data symbol.

The signal so obtained is further applied to a multiplier 110 to bemultiplied by a factor h of the form π/(2m), where m is a positiveinteger greater than one. The signal so obtained is further applied to afrequency modulator 120 performing prior art frequency modulation usinga voltage-controlled or a numerically controlled oscillator, forexample. The phase of the modulated signal is in the form${\phi \left( t^{\prime} \right)} = {\sum\limits_{i}{\alpha_{i}h{\int_{- \infty}^{t - \overset{\_}{iT}}{{g(u)}{u}}}}}$

where h is thus of the form π(2m), m=2, 3, 4, . . . A time reference t′is the start of the data to be transmitted.

The modulated signal is further applied to radio frequency parts 122which can be implemented according to the prior art. It is an advantageof the invention that the radio frequency parts of the GSM system, forexample, can be used as the radio frequency parts, although when themodulation method of the invention is used and m is given a value 2, thedata rate T can be doubled as compared with the GSM system. Themodulated RF signal can be expressed in the form${x\left( t^{\prime} \right)} = {\sqrt{\frac{2E_{c}}{T}}\cos \left( {{2\pi \quad f_{0}t^{\prime}} + {\phi \left( t^{\prime} \right)} + \phi_{0}} \right)}$

where E_(c) is the energy of a modulating symbol, f₀ is a centrefrequency and φ₀ is a random phase which is constant for a period of oneburst. In the radio frequency parts, a C-class amplifier can be used,which is a significant advantage particularly as far as portableterminals are concerned.

The signal is applied from the.radio frequency parts to an antenna 124.

As the transfer function of the filter 108, a raised cosine-typefunction, for example a root raised cosine RRC, can also beadvantageously selected.

FIG. 2 illustrates a second embodiment of the invention. In thisembodiment, no filter exists after the encoder. In other respects, thesolution is similar to the above-described solution.

FIG. 3 illustrates a third alternative embodiment of the invention. Inthis embodiment, the voltage-controlled oscillator of FIG. 1 is replacedby an integrator 300 and a phase modulator 302, from which the signal isfurther applied to the radio frequency parts. In other respects, thesolution is similar to the one described in connection with FIG. 1.

FIG. 4 shows an example of a feasible state diagram of the modulationmethod of the invention, when m=2. Transitions of the state diagram forma unit circle, since a modulation method with a constant amplitude is inquestion and origins and terminals of transitions are indicated by dotson the unit circle. The dots are π/4 phase difference away from oneanother in accordance with the h selected.

FIG. 5 shows an example of a feasible phase trajectory of the modulationmethod of the invention, when m=2. Naturally, the phase trajectorydepends on the symbols to be transmitted, the example showing oneexample thereof. During each symbol, the phase thus changes π/4 ineither direction depending on the symbol. In the figure, a solid lineindicates the phase trajectory generated by the arrangement of FIG. 2,in which phase trajectory no filtering exists. Phase changes are in thatcase sharp, and the frequency ;spectrum then becomes wider. A dottedline indicates the phase trajectory generated by the arrangement of FIG.1, in which phase trajectory phase changes are not that sharp because ofthe filtering. In that case, the frequency spectrum is narrower.

As compared with MSK, for example, in which the phase change is π/2 ineither direction, the method of the invention is more sensitive toerrors occurring on the transmission path because of smaller phasedifferences. However, it is a significant advantage of the method of theinvention that because of smaller frequency changes, the frequency bandrequired becomes narrower during transmission of the same data rate.Consequently, if the frequency band remains unchanged, the data rate canbe increased.

Although the invention is described above with reference to the exampleaccording to the accompanying drawings, it is obvious that the inventionis not restricted thereto, but it can be modified in a variety of wayswithin the scope of the inventive idea disclosed in the attached claims.

What is claimed is:
 1. A method for modulating a signal, the methodusing continuous phase modulation and comprising encoding and frequencymodulation of a signal to be transmitted, characterized in that theencoding performed is differential encoding and that a signal to betransmitted before the frequency modulation is multiplied by a factor ofthe form π/(2m), where m is a positive integer greater than one.
 2. Amethod as claimed in claim 1, characterized in that after the encoding,the signal to be transmitted is filtered.
 3. A method as claimed inclaim 2, characterized in that the transfer function of a filter followsthe Gaussian distribution.
 4. A method as claimed in claim 2,characterized in that the transfer function of the filter is a raisedcosine-type function.
 5. A method as claimed in claim 1, characterizedin that the frequency modulation is performed using a voltage-controlledoscillator.
 6. A method as claimed in claim 1, characterized in that thefrequency modulation is performed using an integrator and a phasemodulator.
 7. A method as claimed in claim 1, characterized in thatencoded symbols are of the form 0 or 1 and that to the encoded symbols,conversion is performed in which symbols 1 and −1 represent the symbols0 and
 1. 8. An arrangement for modulating a signal to be transmitted,the arrangement comprising an encoder (104) and a frequency modulator(120), characterized in that the encoder (104) is a differential encoderand that before the frequency modulator (120), the arrangement comprisesmeans (110) for multiplying the signal to be transmitted by a factor ofthe form π/(2m), where m is a positive integer greater than one.
 9. Anarrangement as claimed in claim 8, characterized in that the arrangementcomprises a filter (108) which is operationally connected to the outputof the encoder.
 10. An arrangement as claimed in claim 8, characterizedin that the frequency modulator (120) is implemented using avoltage-controlled oscillator.
 11. An arrangement as claimed in claim 8,characterized in that the frequency modulator (120) is implemented usingan integrator (300) and a phase modulator (302).